1. Field of the Invention
The present invention relates to a silicon purification method using a metallic silicon material (a base material made of metallic silicon) as a starting material, and particularly to a silicon purification method that can purify high purity silicon which is suitable for a solar cell material and has a low impurity content of phosphorus (P), oxygen (O), boron (B), carbon (C), iron (Fe), aluminum (Al), calcium (Ca), titanium (Ti) or the like.
2. Background Art
As a conventional silicon purification method, for example, the method described below is known (refer to Japanese Unexamined Patent Application, First Publication No. H10-182134).
In this method, a metallic silicon material (a base material made of metallic silicon) with a predetermined purity is prepared (refer to Step S11 in FIG. 3B) as a starting material, then the metallic silicon material is subjected to, firstly, a reduced-pressure purification and then vacuum refining, whereby P that is an impurity contained in the metallic silicon material is removed by volatilization (refer to Step S12 in FIG. 3B).
Next, the metallic silicon material is subjected to oxidative purification, whereby B or C contained in the metallic silicon which has been purified under reduced-pressure is removed (refer to Step S14 in FIG. 3B).
Next, the metallic silicon material is again subjected to reduced-pressure purification, whereby O contained in the oxidatively purified metallic silicon is removed (refer to Step S15 in FIG. 3B).
Next, the metallic silicon material is subjected to solidification purification (by which a molten metal portion is solidified unidirectionally, and the residual molten metal portion is removed by casting), whereby metal impurities, such as Fe, Al, Ca, and the like, contained in the metallic silicon from which O has been removed are removed (refer to Step S16 in FIG. 3B).
However, silicon used for manufacturing a solar cell demands a low impurity concentration, therefore the impurity concentration of any of, for example, Fe, Al, Ca, Ti, and P needs to be less than 0.1 ppm.
Consequently, depending on the impurity concentration of a starting material, there are cases in which the above conventional silicon purification method cannot produce high-purity silicon that satisfies such a condition.
As a conventional silicon purification method that purifies high-purity silicon which is to be used to manufacture a solar cell, the method described below is known (refer to Japanese Unexamined Patent Application, First Publication No. 2000-327488).
In this method, as a starting material, a metallic silicon material (a base material made of metallic silicon) with a predetermined purity is prepared (refer to Step S11 in FIG. 3B), then the metallic silicon material is subjected to, firstly, a reduced-pressure purification and then vacuum refining, whereby phosphorous that is an impurity contained in the metallic silicon material is removed by volatilization (refer to Step S12 in FIG. 3B).
Next, the metallic silicon material is subjected to solidification purification (by which a molten metal portion is solidified unidirectionally, and the residual molten metal portion is removed by casting), whereby metal impurities, such as Fe, Al, Ca, and the like, contained in the metallic silicon which has been purified under reduced-pressure are removed (refer to Step S13 in FIG. 3B).
Next, the metallic silicon material is subjected to oxidative purification, whereby B or C contained in the solidification-purified metallic silicon is removed (refer to Step S14 in FIG. 3B).
Next, the metallic silicon material is again subjected to reduced-pressure purification, whereby O contained in the oxidatively purified metallic silicon is removed (refer to Step S15 in FIG. 3B).
Finally, the metallic silicon material is again subjected to solidification purification, whereby metal impurities are removed (refer to Step S16 in FIG. 3B).
Furthermore, as a conventional method for the above solidification purification, for example, a method is known in which a raw metal is fed into a water-cooled crucible of an electron beam melting furnace and fully melted by irradiating the entire surface of the raw metal with an electron beam, then the irradiation range of the electron beam is narrowed by controlling a deflecting coil (refer to Japanese Patent No. 3848816).
However, the above conventional silicon purification method has a problem in that time-consuming reduced-pressure purification needs to be conducted twice, and, in order to reliably purify higher-purity silicon, time-consuming solidification purification need to be conducted twice, whereby the number of processes increases and the time necessary for silicon purification lengthens.
If a metallic silicon material with an extremely low impurity concentration is used as a starting material, high purity silicon can be obtained with a single solidification purification process, but the cost of the starting material increases.